localedef: Fix handling of empty mon_decimal_point (Bug 28847)

localedef: Fix handling of empty mon_decimal_point (Bug 28847)

The handling of mon_decimal_point is incorrect when it comes to
handling the empty "" value.  The existing parser in monetary_read()
will correctly handle setting the non-wide-character value and the
wide-character value e.g. STR_ELEM_WC(mon_decimal_point) if they are
set in the locale definition.  However, in monetary_finish() we have
conflicting TEST_ELEM() which sets a default value (if the locale
definition doesn't include one), and subsequent code which looks for
mon_decimal_point to be NULL to issue a specific error message and set
the defaults. The latter is unused because TEST_ELEM() always sets a
default.  The simplest solution is to remove the TEST_ELEM() check,
and allow the existing check to look to see if mon_decimal_point is
NULL and set an appropriate default.  The final fix is to move the
setting of mon_decimal_point_wc so it occurs only when
mon_decimal_point is being set to a default, keeping both values
consistent. There is no way to tell the difference between
mon_decimal_point_wc having been set to the empty string and not
having been defined at all, for that distinction we must use
mon_decimal_point being NULL or "", and so we must logically set
the default together with mon_decimal_point.

Lastly, there are more fixes similar to this that could be made to
ld-monetary.c, but we avoid that in order to fix just the code
required for mon_decimal_point, which impacts the ability for C.UTF-8
to set mon_decimal_point to "", since without this fix we end up with
an inconsistent setting of mon_decimal_point set to "", but
mon_decimal_point_wc set to "." which is incorrect.

Tested on x86_64 and i686 without regression.
Reviewed-by: Florian Weimer <fweimer@redhat.com>